The present invention relates to a flash device, and more particularly to a flash device which is suitable for being incorporated in a lens-fitted photo film unit that contains a roll of photo filmstrip and is provided with a simple photographic mechanism including a taking lens.
When the subject brightness is so low that a proper exposure would not be provided without any artificial illumination, a flash device is often used to project light toward the subject synchronously with the shutter release. Since compact cameras and lens-fitted photo film units have an inexpensive lens system of a relative large f-number, most of them are provided with a built-in flash device. To make a flash photography, it is necessary to charge the main capacitor up to the set voltage prior to the shutter release. The conventional flash devices start charging in response to an actuation of a flash charge switch.
A flash circuit has recently been known, for example from JPA 7-122389, wherein once a flash charge switch is turned on the main capacitor continues to be charged even after the flash charge switch is turned off. The flash circuit stops charging while the main capacitor is at the set charge voltage. Hereinafter, this type of flash device will be referred to as an autostop flash circuit.
An example of autostop flash circuit is shown in FIG. 24, which has fundamentally the same configuration as that disclosed in JPA 7-122389, except some minor differences. In the flash circuit of FIG. 24, when a flash charge switch 200 is turned on, an oscillation transistor 201 is activated and starts oscillating due to positive feedback of an oscillation transformer 202. The oscillation causes an increase in a primary current that flows through a primary coil 202a, i.e. a collector current that flows to the collector of the oscillation transistor 200. As a result, an electromotive force induces a current through a secondary coil 202b, and the current charges a main capacitor 204 through a rectifying diode 203.
Since the collector current flows through the oscillation transistor 201, a latching transistor 205 is turned on. Thereafter when the increment of the primary current goes down, a back electromotive force is generated in the secondary coil 202b, and the current fed back from the oscillation transformer 202 to the oscillation transistor 201, i.e. base current of the oscillation transistor 201, begins to decrease. However, a voltage from a battery 206 is applied to the base of the oscillation transistor 201 through the latching transistor 205 as being in the ON state, the oscillation transistor 201 is not completely turned off. Therefore, the primary current starts flowing again, thereby the oscillation transistor 201 continues to oscillate and charge the main capacitor 204.
A Zener diode 207 with a Zener voltage of 300V is provided for starting conducting a Zener current when the main capacitor 204 is charged up to a set voltage of 300V. Because a base current is applied to a base of a stopping transistor 208 due to the Zener current, the stopping transistor 208 is turned on. When the stopping transistor 208 is turned on, the emitter and the base of the oscillation transistor 201 are connected to each other, so that the oscillation transistor 201 is completely turned off, and thus the latching transistor 205 is turned off. In this way, the oscillation stops to stop charging the main capacitor 204 when the main capacitor 204 is charged up to the set voltage.
In the above autostop flash circuit, since the charge voltage of the main capacitor is applied to the Zener diode to conduct the Zener current for activating the stopping transistor when the charge voltage reaches the set value, the Zener diode must have a high Zener voltage, e.g. 300V, in correspondence with the set charge voltage. As the Zener diode with high Zener voltage is expensive, it raises the cost of the flash circuit. In addition, the conventional autostop flash circuit requires a lot of space for mounting various elements as above which are necessary for the automatic continuation and stopping of charging.
Meanwhile, a lens-fitted photo film unit having a flash device incorporated therein is widely known. Since the flash device for the lens-fitted photo film unit is required to be inexpensive and compact, the flash switch is constituted of a metal blade and contact chips formed on a flash circuit board such that the metal blade is brought into contact with the contact chips through a manual operation member, to close the flash circuit and thus cause the flash circuit to start charging. Conventionally, the manual operation member is a push button or a sliding button. Some of the conventional manual operation members are provided with a fastening mechanism by which the metal blade is kept in contact with the contact chips once the operation member is operated for a moment. Others are designed to bring the metal blade into contact with the contact chips only while the photographer operates the manual operation member.
The flash device of the lens-fitted photo film unit has been provided with a light emission element for indicating completion of charging the flash device. In the conventional lens-fitted photo film unit, the light emission element is placed behind an indication window formed through a rear wall of the lens-fitted photo film unit, or is placed in connection to a light guide that conducts light from the light emission element to a view field of a viewfinder.
Where the charge switch operation device is provided with the fastening device, the flash device keeps charging so long as the operation device is in the ON position. If the operation device is left in the charging position after the photography is terminated, the battery is wasted. As a result, the battery runs down and it becomes impossible to use the flash device before all of the available exposures are carried out. Such trouble will be prevented if only the photographer checks the position of the operation member or the light from the light emission element at the conclusion of photography. However, according to the configurations of the conventional lens-fitted photo film units, it is not easy to know the switching condition of the operation member at a glance, or the charge condition indicating light is not visible from the outside of the lens-fitted photo film unit. Therefore, the photographer can fail to reset the charge switch to the OFF position.
In view of the foregoing, a prime object of the present invention is to provide an autostop flash device which cuts the cost and space without lowering reliability and stability.
A further object of the present invention is to provide a flash device which is effective to prevent the photographer from forgetting to turn off the flash charge switch, and is also suitable especially for use in a lens-fitted photo film unit.
In a flash device comprising an oscillation circuit that starts oscillating when a flash charge switch is turned on, wherein the oscillation circuit is comprised of an oscillation transformer having a primary coil connected to a power source and a secondary coil connected to a main capacitor, and the secondary coil is inductively coupled to the primary coil such that a high voltage current is induced in the secondary coil while the oscillation circuit oscillates, and that the main capacitor is charged with the high voltage current up to a set charge voltage,
the present invention is comprised of a tap point located at an intermediate position of the secondary coil, the tap point having a potential that changes proportionally to the charge voltage across the main capacitor; a Zener diode connected to the tap point, to conduct a Zener current when the potential at the tap point reaches a value that corresponds to the set charge voltage of the main capacitor; and a stopping transistor activated by the Zener current to stop the oscillation circuit from oscillating and thus stop charging the main capacitor when the main capacitor reaches the set charge voltage.
The flash device according to the invention makes it possible to use an inexpensive Zener diode with a low Zener voltage, so that it is possible to cut the cost of the flash device.
By charging a stopping capacitor with the Zener current that flows when the main capacitor reaches the set charge voltage and then applying current discharged from the stopping capacitor to the stopping transistor through a resistor, the stopping transistor keeps operating for a predetermined time. It ensures stopping charging the main capacitor, and also prevents unexpected interruption of charging that may be caused by noises.
Moreover, a temperature coefficient of a forward voltage of a rectifying diode that blocks current flowing from the tap point to the Zener diode should have an opposite polarity to a temperature coefficient of the Zener voltage of the Zener diode, such that the Zener diode and the rectifying diode form a mutual temperature compensating circuit. Thereby, the main capacitor is charged up to the constant set voltage without being affected by environmental temperature or the like.
To achieve the second object in a flash device comprising a flash circuit and a flash charge switch for charging the flash circuit, the present invention is characterized by comprising an indication device for indicating completion of charging the flash circuit, the indication device protruding outside when the flash charge switch is turned on.
As the indication device for indicating the completion of charging the flash circuit protrudes outside when the flash charge switch is turned on, it is easy to check if the flash charge switch is in the ON state or not. Accordingly, the present invention is effective to remind the photographer to turn off the charge switch when the flash circuit needs not charging. Thus, the flash device of the present invention solves the above described problem of wasting the battery and making the flash device useless for the following photography.